1. Field of the Invention
This invention relates to an improved automated storage library. More particularly, the invention is an automated storage library including one or more robotic accessors which move upon the surface of a horizontal plane including the openings to storage cells.
2. Description of the Related Art
Modern computers require a host processor including one or more central processing units and a memory facility. The processor manipulates data stored in the memory according to instructions provided to it. The memory must therefore be capable of storing data required by the processor and transferring that data to the processor at a rate capable of making the overall operation of the computer feasible. The cost and performance of computer memory is thus critical to the commercial success of a computer system.
Because today's computers require large quantities of data storage capacity, computer memory is available in many forms. A fast but expensive form of memory is main memory, typically comprised of microchips. Other available forms of memory are known as peripheral storage devices and include magnetic direct access storage devices (DASD), magnetic tape storage devices, and optical recording devices. These memory devices store data on storage media, such as disks and tapes. Peripheral storage devices have a greater storage density and lower cost than main memory, but fail to provide the same performance. For example, the time required to properly position a tape or disk beneath a read/write mechanism of a drive cannot compare with the rapid, purely electronic data transfer rate of main memory.
It is inefficient to store all of the data in a computer system on a single type of memory device. It is too costly to store all of the data in main memory and performance is reduced too much to store all of the data on peripheral storage devices. Thus, a typical computer system includes both main memory and one or more types of peripheral storage devices arranged in a data storage hierarchy. The data storage hierarchy arrangement is tailored to the performance and cost requirements of the user. In such a hierarchy, main memory is often referred to as primary data storage, the next level of the hierarchy is often to referred to as secondary data storage, and so on. Generally, the highest level of the hierarchy has the lowest storage density capability, highest performance and highest cost. As one proceeds down through the hierarchy, storage density generally increases, performance generally decreases, and cost generally decreases. By transferring data between different levels of the hierarchy as required, the cost of memory is minimized and performance is maximized. Data is thus stored in main memory only so long as it is expected to be required by the processor. The hierarchy may take many forms, include any number of data storage or memory levels, and may be able to transfer data directly between any two distinct memory levels. The transfer of data may employ I/O channels, controllers, or cache memories as is well known in the art.
The need for memory is expanding. Data to be stored may include coded data and uncoded data, such as images. Images may be included in engineering drawings, financial and insurance documents, medical charts and records, etc. Images can take many forms, and therefore cannot be encoded into the binary 0's and 1's of computers as easily and compactly as text. Most of the world's data, particularly image data, is still stored on paper. The cost of filing, storing, and retrieving such paper documents including image data is escalating rapidly. It is no longer acceptable to maintain rooms or warehouses stocked full of documents which must be retrieved manually when access thereto is required. Optical scanners are now capable of converting images into machine readable form for storage on peripheral storage devices, but the storage space required for the image data--although significantly less than that required for paper documents--is still quite large. Numerous disks or tapes are required for most business applications. Automated storage libraries have thus been developed to manage the storage of such disks and tapes.
Automated storage libraries include a plurality of storage cells for retaining removable data storage media, such as magnetic tapes, magnetic disks, or optical disks, a robotic accessor mechanism, and one or more internal peripheral storage devices. Each data storage medium may be contained in a cassette or cartridge housing for easier handling by the accessor. The accessor operates on command to transfer the data storage media between the storage cells and the internal peripheral storage devices without manual assistance. Once a data storage medium is mounted in an internal peripheral storage device, data may be written to or read out from that medium for as long as the system so requires. Data is stored on a medium in the form of one or more files, each file being a logical data set. The internal peripheral storage devices and storage cells may be considered distinct levels of a data storage hierarchy. In addition, data storage media in shelf storage (i.e. not in the storage cells, but instead outside the reach of the robotic accessor without manual intervention) may be considered yet another level of a data storage hierarchy.
Several automated storage libraries are known. IBM Corporation introduced the 3850 Mass Storage Subsystem for the storage and retrieval of magnetic tape modules in the 1970's. This library stored tape modules in a stationary, honeycombed array of storage cells. The array was planar and oriented vertically. Tape modules were moved horizontally into and out of the storage cells and tape drives by the accessor.
More recently, several firms have introduced automated storage libraries for magnetic tape cartridges and optical disks. These libraries include numerous variations in configuration, but always arrange the storage cells in a vertical array. For example, U.S. Pat. No. 4,654,727 discloses a magnetic tape cartridge library in which columns of stationary storage cells are arranged in a generally circular array. The openings of the storage cells face the center of the array, where the accessor is located. Although the storage cells tilt downward from their openings, the openings are arranged in one or more vertical planes.
Another magnetic tape library is disclosed in U.S. Pat. Nos. 4,864,438, and 4,864,511. The configuration of this library is similar to that disclosed in U.S. Pat. No. 4,654,727, except that an accessor may access two stationary arrays of storage cells. The arrays are generally circular and concentric. The accessor is located between the arrays. The openings of the inner array face outward and the openings of the outer array face inward. The openings of the storage cells are again arranged in one or more vertical planes.
Yet another magnetic tape library is disclosed in U.S. Pat. No. 4,271,440. This library includes storage cells arranged in circular arrays which can be rotated to bring a particular cell to a position of close proximity to an accessor. Again, the openings of the storage cells are arranged vertically. Additional library configurations are known, but the openings of the storage cells are always arranged vertically. An example of a magnetic tape library with such an arrangement can be found in U.S. Pat. No. 5,015,139. Examples of optical disk libraries with such an arrangement can be found in U.S. Pat. Nos. 4,271,489, 4,527,262, 4,614,474, 4,608,679, 4,766,581, and in the IBM 3995 Optical Library Dataserver.
Several factors are known to affect library efficiency, including storage capacity, expandability, moving mass, connectivity, failure points, and flexibility. Storage capacity is the amount of data that may be contained within a library. Expandability is the ability to increase the storage capacity of a library. For example, the storage capacity can be increased by adding additional storage cells, if possible. Moving mass is the amount of mass which must be moved during the transfer of storage media. Generally, a reduction in moving mass increases the speed of movement and improves reliability. Connectivity is the freedom to transfer storage media between specific storage cells and peripheral storage devices. Ideally, a library can transfer any storage medium therein to any peripheral storage device therein. Failure points are the areas where the library is likely to fail. Ideally, a library will contain no single points of failure, thereby improving reliability. Flexibility is the ability to change certain library characteristics, such as the number of accessors or grippers per accessor.
Existing libraries fail to achieve adequate efficiency because of one or more of the aforementioned factors. For example, the libraries disclosed in U.S. Pat. Nos. 4,654,727, 4,864,438 and 4,864,511 each have a robotic accessor mechanism which is a single point of failure relating to all or a portion of the storage media therein. In addition, expandability is inhibited because the vertically arranged storage cells completely surround the accessor. The library disclosed in the '438 and '511 patents can be expanded by interconnecting several silos (a silo is a single accessor and its surrounding storage cells) with mechanical passthru mechanisms. Unfortunately, the passthru mechanism is relatively slow and reduces reliability. Also, expansion can only occur in silo increments, thereby reducing floor space utilization in many installations.
Other libraries may improve upon some of the aforementioned factors, but at the expense of other such factors. To the extent, if any, that machines used outside of the data processing industry (such as jukeboxes) are analogous, the same tradeoffs apply. A heretofore unrecognized and unresolved problem is thus the optimization of the aforementioned factors in automated storage libraries.